BBC Radio 4 long wave reception problems etc. - Page 25 - UK Vintage Radio Repair and Restoration Discussion Forum

G0HZU_JMR · 2nd Feb 2019, 11:32 pm

When the Droitwich transmitter is behaving itself the 1kHz pips look to be quite good on 198kHz here. I'm really just trying to describe the difference in harmonic distortion levels (of the time pips) seen on the analyser data between times when the transmitter has the sibilance issue and when it is working normally. When it is working normally the pips look fairly decent to me and I get 0.7% THD on the demodulated pips. I guess it's up to the individual to decide if this is a fairly acceptable sine wave or not. When the sibilance issue was present the THD shot up to 10%. That difference is all I'm really trying to describe

Obviously, there's not much point in the BBC trying for Hifi levels of distortion for a LW AM transmitter but I can't see any significant odd order harmonics (the components of a square wave) when I look on the analyser when the transmitter is behaving itself without the sibilance issue.

I'm only a few miles from the transmitter and I'm doing all my tests and demodulation in the digital domain using data from the spectrum analyser and I'm doing it in daylight hours to minimise propagation effects.

Obviously, it would be risky to try and assess the transmitter quality (wrt the time pips) with a low cost domestic LW AM receiver as the rapidly rising pip tones would contribute to (AF) AGC induced distortion, distortion in the IF amplifier and detector and maybe even distortion in the AF stages.

Refugee · 3rd Feb 2019, 1:13 am

The pips sounded pretty clean this evening on FM so they have fixed the problem and by the sound of it the transmitter was not at fault due to it being on both FM and LW.

Pfraser · 3rd Feb 2019, 8:06 pm

Quote:

Originally Posted by Refugee

The pips sounded pretty clean this evening on FM so they have fixed the problem and by the sound of it the transmitter was not at fault due to it being on both FM and LW.

This opens up a byway of enquiry.

I once had a lively discussion with a schoolfriend regarding the origin of the BBC Radio 2 signal for the Midlands, on 693kHz. I was robust in my assertion that it came from Droitwich; my friend was equally firm in stating Sutton Coldfield.

Now...it later transpired that he was correct, in part. For whilst the radiated signal did indeed come from Droitwich, the programme feed was routed via Sutton Coldfield.

So...has the venerable 198kHz behemoth been falsely maligned?

Has there been a fault further back in the chain?

Refugee · 4th Feb 2019, 1:34 am

I certainly have my doubts about there being anything wrong with either of the final transmitters due to the problem being on both 198Khz and FM.

G0HZU_JMR · 5th Feb 2019, 12:05 am

I had a go at filming the time pips on my analyser last night when there wasn't any obvious sibilance. See below for the video. The 1kHz pips still look quite good but I'm not convinced that the transmission is quite as clean as usual. It's a lot better than it was last Friday but there is still some spectral spreading beyond +/- 5kHz when the announcer says words with an 's' at the end. So maybe there is still a hint of sibilance? Maybe this level of spreading is insignificant, but it normally looks a bit better than this.

The pips do look like a sine wave when the captured analyser data is demodulated and you can see on the analyser that the main modulation sidebands are at 1kHz and there are no visible sidebands at odd order multiples of 1kHz. So I don't think there is any kind of filtered square wave being sent. It looks like they are at least trying for a clean sine wave at 1kHz.

If you look closely, the main carrier dips on modulation peaks and I recall that someone on here mentioned that they dip the carrier power like this to improve efficiency.

I've got about 5 minutes of captured IQ data from last Friday when the sibilance was bad and I could post up what the pips look like when the transmission has this problem?

Sorry about the dodgy quality of the short video but I hope it is interesting.

https://youtu.be/F4xJ1wX7Z4w

paulsherwin · 5th Feb 2019, 12:16 am

The transmitted signal should stay within its transmission bandwidth even if distortion has been added earlier in the distribution chain, though I don't have an explanation and agree it's very puzzling.

To complicate things further, I understand the distribution circuit to Droitwich R4 is some arcane ISDN arrangement separate from the standard BBC Nicam circuits. This dates back to the 80s when 198kHz was still expected to have a military / civil defence role in a national emergency.

G8HQP Dave · 5th Feb 2019, 11:58 am

Quote:

Originally Posted by Hartley118

The pips sound magnificent on DAB.

But always slightly late?

G0HZU_JMR · 10th Feb 2019, 1:18 am

Quote:

The transmitted signal should stay within its transmission bandwidth even if distortion has been added earlier in the distribution chain, though I don't have an explanation and agree it's very puzzling.

I had a bit of free time today to play with a larger (passive) antenna and this antenna works across the whole SW band. So I could compare BBC R4 against other stations.

I compared it against 1053kHz Talksport and both were received with about the same signal strength on the analyser (within a couple of dB). However, the Talksport transmission stays within its +/-5kHz bandwidth without any splashy IMD beyond this bandwidth. The BBC R4 transmission on 198kHz showed a fair bit of splashy IMD beyond +/- 5kHz today and I think it was slightly worse than my earlier video and the time pips had a bit more distortion on them today. So it is as if the signal quality has degraded a bit since it was 'fixed' last week.

dave walsh · 10th Feb 2019, 4:09 am

Do members of the public ever notice this sibulant fault state? I've never seen anything in the Radio Times for example!

Dave W

G0HZU_JMR · 11th Feb 2019, 12:39 am

I think it would have been noticed by lots of listeners on the day it was sounding poor (Friday 1/2/2019?) as it didn't sound good at all on that day.

I did search the internet for problem reports for R4 on 198kHz and it had already been reported as a problem on at least one site for that day. I can't remember if it said it was noisy or if it said it was distorted but I think it was this site here that reported the problem:

http://www.radioandtvhelp.co.uk/inte...radio/am_radio

So it does look like the problem gets noticed and the time of the report appeared to be just before it was fixed on that Friday afternoon. However, I can't actually find that report today so maybe they delete them after a while.

I recorded the pips again today and demodulated them directly in the analyser and the analyser can show the modulation envelope for each pip. Each pip looks to be about 100ms long and the final pip is about 500ms long. They seem to be spaced exactly 1 second apart on the leading edge of each pip. The pip envelope has a rise and fall time of a few ms.

dave walsh · 11th Feb 2019, 12:58 am

There was-or is, some kind of a relay TX here in Bexhill. A few years ago, I bought a Murphy set from a chap who lived nearby. He had a huge coil of telephone extension cable in the hallway and said that it was the only way that he could stop Radio 4 harmonically resonating and breaking through on his calls-a bit like Taxi's and stage amps!.

Dave W

Ian - G4JQT · 28th Dec 2020, 10:06 am

Oh dear. 198 sounds dreadful this morning, and splatter is confirmed on SDR. Is it just Droitwich or Westergen and Burghead?

Ian - G4JQT · 28th Dec 2020, 10:07 am

Oh, fixed just after I posted this! Well done.

Ted Kendall · 28th Dec 2020, 10:32 am

I think they've worked out which plug to kick by now...

lesmw0sec · 28th Dec 2020, 10:41 am

Yes! Turn on the R1155 for the news and thought the set had expired. Engineering standards are now approaching the low of the material transmitted...

Station X · 28th Dec 2020, 1:11 pm

Threads merged.

rojosuiza · 10th Jul 2022, 5:25 pm

I have been checking, but all my radios are in good working order.

Since this morning this frequency is distortion only... Is it the same in the UK? Is anything known about the reason for this? Does anybody know, if the malfunction has been signaled to the BBC already?

paulsherwin · 10th Jul 2022, 5:35 pm

Very longstanding issue. Arqiva who operate the transmitter appear to understand what the problem is, but they never implement a permanent fix, presumably for financial reasons. The BBC have been trying to close 198kHz for at least a decade, but there is political resistance. See https://www.vintage-radio.net/forum/...d.php?t=136710

Aub · 10th Jul 2022, 5:59 pm

Yes, I noticed the sibilance this morning. Don't know if it's fixed yet.

Cheers

Aub

rojosuiza · 10th Jul 2022, 6:01 pm

A long story, indeed.

The reception of 198kHz here in Amsterdam has been quite good the last few months. Me very happy.

Only now it is suddenly very, very bad, the transmitter seems to have a serious problem.

How does one find out if they are working on it?

2nd Feb 2019, 11:32 pm	#481
G0HZU_JMR Dekatron Join Date: Sep 2010 Location: Cheltenham, Gloucestershire, UK. Posts: 3,077	Re: BBC Radio 4 198 kHz long wave sibilance. When the Droitwich transmitter is behaving itself the 1kHz pips look to be quite good on 198kHz here. I'm really just trying to describe the difference in harmonic distortion levels (of the time pips) seen on the analyser data between times when the transmitter has the sibilance issue and when it is working normally. When it is working normally the pips look fairly decent to me and I get 0.7% THD on the demodulated pips. I guess it's up to the individual to decide if this is a fairly acceptable sine wave or not. When the sibilance issue was present the THD shot up to 10%. That difference is all I'm really trying to describe Obviously, there's not much point in the BBC trying for Hifi levels of distortion for a LW AM transmitter but I can't see any significant odd order harmonics (the components of a square wave) when I look on the analyser when the transmitter is behaving itself without the sibilance issue. I'm only a few miles from the transmitter and I'm doing all my tests and demodulation in the digital domain using data from the spectrum analyser and I'm doing it in daylight hours to minimise propagation effects. Obviously, it would be risky to try and assess the transmitter quality (wrt the time pips) with a low cost domestic LW AM receiver as the rapidly rising pip tones would contribute to (AF) AGC induced distortion, distortion in the IF amplifier and detector and maybe even distortion in the AF stages. __________________ Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 2nd Feb 2019 at 11:54 pm.

3rd Feb 2019, 1:13 am	#482
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: BBC Radio 4 198 kHz long wave sibilance. The pips sounded pretty clean this evening on FM so they have fixed the problem and by the sound of it the transmitter was not at fault due to it being on both FM and LW.

4th Feb 2019, 1:34 am	#484
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: BBC Radio 4 198 kHz long wave sibilance. I certainly have my doubts about there being anything wrong with either of the final transmitters due to the problem being on both 198Khz and FM.

5th Feb 2019, 12:05 am	#485
G0HZU_JMR Dekatron Join Date: Sep 2010 Location: Cheltenham, Gloucestershire, UK. Posts: 3,077	Re: BBC Radio 4 198 kHz long wave sibilance. I had a go at filming the time pips on my analyser last night when there wasn't any obvious sibilance. See below for the video. The 1kHz pips still look quite good but I'm not convinced that the transmission is quite as clean as usual. It's a lot better than it was last Friday but there is still some spectral spreading beyond +/- 5kHz when the announcer says words with an 's' at the end. So maybe there is still a hint of sibilance? Maybe this level of spreading is insignificant, but it normally looks a bit better than this. The pips do look like a sine wave when the captured analyser data is demodulated and you can see on the analyser that the main modulation sidebands are at 1kHz and there are no visible sidebands at odd order multiples of 1kHz. So I don't think there is any kind of filtered square wave being sent. It looks like they are at least trying for a clean sine wave at 1kHz. If you look closely, the main carrier dips on modulation peaks and I recall that someone on here mentioned that they dip the carrier power like this to improve efficiency. I've got about 5 minutes of captured IQ data from last Friday when the sibilance was bad and I could post up what the pips look like when the transmission has this problem? Sorry about the dodgy quality of the short video but I hope it is interesting. https://youtu.be/F4xJ1wX7Z4w __________________ Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 5th Feb 2019 at 12:21 am.

5th Feb 2019, 12:16 am	#486
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: BBC Radio 4 198 kHz long wave sibilance. The transmitted signal should stay within its transmission bandwidth even if distortion has been added earlier in the distribution chain, though I don't have an explanation and agree it's very puzzling. To complicate things further, I understand the distribution circuit to Droitwich R4 is some arcane ISDN arrangement separate from the standard BBC Nicam circuits. This dates back to the 80s when 198kHz was still expected to have a military / civil defence role in a national emergency.

10th Feb 2019, 4:09 am	#489
dave walsh Dekatron Join Date: Jan 2006 Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex. Posts: 5,817	Re: BBC Radio 4 198 kHz long wave sibilance. Do members of the public ever notice this sibulant fault state? I've never seen anything in the Radio Times for example! Dave W

11th Feb 2019, 12:39 am	#490
G0HZU_JMR Dekatron Join Date: Sep 2010 Location: Cheltenham, Gloucestershire, UK. Posts: 3,077	Re: BBC Radio 4 198 kHz long wave sibilance. I think it would have been noticed by lots of listeners on the day it was sounding poor (Friday 1/2/2019?) as it didn't sound good at all on that day. I did search the internet for problem reports for R4 on 198kHz and it had already been reported as a problem on at least one site for that day. I can't remember if it said it was noisy or if it said it was distorted but I think it was this site here that reported the problem: http://www.radioandtvhelp.co.uk/inte...radio/am_radio So it does look like the problem gets noticed and the time of the report appeared to be just before it was fixed on that Friday afternoon. However, I can't actually find that report today so maybe they delete them after a while. I recorded the pips again today and demodulated them directly in the analyser and the analyser can show the modulation envelope for each pip. Each pip looks to be about 100ms long and the final pip is about 500ms long. They seem to be spaced exactly 1 second apart on the leading edge of each pip. The pip envelope has a rise and fall time of a few ms. __________________ Regards, Jeremy G0HZU

11th Feb 2019, 12:58 am	#491
dave walsh Dekatron Join Date: Jan 2006 Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex. Posts: 5,817	Re: BBC Radio 4 198 kHz long wave sibilance. There was-or is, some kind of a relay TX here in Bexhill. A few years ago, I bought a Murphy set from a chap who lived nearby. He had a huge coil of telephone extension cable in the hallway and said that it was the only way that he could stop Radio 4 harmonically resonating and breaking through on his calls-a bit like Taxi's and stage amps!. Dave W

28th Dec 2020, 10:06 am	#492
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,323	Radio Four sibilance - again Oh dear. 198 sounds dreadful this morning, and splatter is confirmed on SDR. Is it just Droitwich or Westergen and Burghead? Attached Thumbnails

28th Dec 2020, 10:07 am	#493
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,323	Re: Radio Four sibilance - again Oh, fixed just after I posted this! Well done.

28th Dec 2020, 10:32 am	#494
Ted Kendall Dekatron Join Date: Jul 2008 Location: Kington, Herefordshire, UK. Posts: 3,671	Re: Radio Four sibilance - again I think they've worked out which plug to kick by now...

28th Dec 2020, 10:41 am	#495
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,507	Re: Radio Four sibilance - again Yes! Turn on the R1155 for the news and thought the set had expired. Engineering standards are now approaching the low of the material transmitted...

28th Dec 2020, 1:11 pm	#496
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Radio 4 198 sibilance Threads merged. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

10th Jul 2022, 5:25 pm	#497
rojosuiza Tetrode Join Date: Mar 2018 Location: Amsterdam, Netherlands Posts: 68	198kHz - horrible distortion I have been checking, but all my radios are in good working order. Since this morning this frequency is distortion only... Is it the same in the UK? Is anything known about the reason for this? Does anybody know, if the malfunction has been signaled to the BBC already?

10th Jul 2022, 5:35 pm	#498
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: 198kHz - horrible distortion Very longstanding issue. Arqiva who operate the transmitter appear to understand what the problem is, but they never implement a permanent fix, presumably for financial reasons. The BBC have been trying to close 198kHz for at least a decade, but there is political resistance. See https://www.vintage-radio.net/forum/...d.php?t=136710

10th Jul 2022, 5:59 pm	#499
Aub Nonode Join Date: Mar 2005 Location: Nuneaton, Warwickshire, UK. Posts: 2,039	Re: 198kHz - horrible distortion Yes, I noticed the sibilance this morning. Don't know if it's fixed yet. Cheers Aub __________________ Life's a long song, but the tune ends too soon for us all.

10th Jul 2022, 6:01 pm	#500
rojosuiza Tetrode Join Date: Mar 2018 Location: Amsterdam, Netherlands Posts: 68	Re: 198kHz - horrible distortion A long story, indeed. The reception of 198kHz here in Amsterdam has been quite good the last few months. Me very happy. Only now it is suddenly very, very bad, the transmitter seems to have a serious problem. How does one find out if they are working on it?